For all my longer works I write chapter outlines so I can have the pleasure of departing from them later on.
— Garth Nix
Regardless of whether you’re writing a longer work or a short piece, you have to start with an outline. The second step is to write a brief synopsis of your story detailing its beginning, middle, and end. Even if you end up deviating from this, as Nix does, you’ll know that you’ve at least thought through your original idea and that it has a logical progression or arc.
Then, write a master plot list for each of your characters, beginning with the main one and working your way down the pecking order. This list substantiates why those characters are in the story.
Now the fun begins. Before you write one word of the actual work, you need to create a backstory for every character, even if you don’t use it. Flesh out who these people are and how they interconnect in the story, and identify their motivations and how they will affect the plot.
Once you’ve finished your piece (or if it’s large, at the end of each chapter), go back and compare what you’ve done to these lists. If you’ve strayed off course, figure out why and whether or not your new idea is better than your original.
One word for mastery: Consistency.